Switch greeter from tuigreet to regreet

- Replace greetd-tuigreet with greetd-regreet + cage in packages.txt
- Add greetd/config.toml and greetd/regreet.toml (Materia-dark theme)
- Update install.sh to deploy greetd configs to /etc/greetd/
- Fix powermenu.sh awk parsing (use $NF instead of $2 for icon glyphs)

scripts/powermenu.sh

@@ -8,7 +8,7 @@ CHOICE=$(printf "  Logout\n  Restart\n  Sleep\n  Shutdown" \
            --hide-scroll \
            --no-actions \
            --insensitive \
-    | awk '{print $2}')
+    | awk '{print $NF}')
 
 case "$CHOICE" in
     Logout)   niri msg action quit ;;